The present invention relates to the process of producing ammonium dimolybdate for conversion to pure molybdenum oxide or other pure chemicals from molybdenite concentrates and more particularly to a process of producing chemical grade ammonium dimolybdate for conversion to chemical grade molybdenum oxide through a process that includes the pressure oxidation of low-grade molybdenite concentrates.
Extraction of molybdenum from molybdenite-containing materials by an aqueous process has been the subject of study for over 50 years. In 1952, E. S. Usataya1 reported on the oxidation of molybdenite in water solutions. He found that in neutral, acidic, or weakly alkaline solutions the decomposition products precipitate on the molybdenite surface and protect the molybdenite from further oxidation. He found that strong bases and strong oxidizing agents impede the formation of the protective layers. Increasing temperature accelerated the oxidation rate in alkaline solutions, but up to 60xc2x0 C. had no effect in acidic solutions.
1 Usataya, E. S., xe2x80x9cOxidation of molybdenite in water solutions,xe2x80x9d Zapiski Vsesoyuz Mineral Obshschestva, v 81, 298-303 (1952). 
A Japanese patent2 was issued for oxygen pressure oxidation (POX) of molybdenite in 1962. The example in this patent leached a 55.5% Mo, 36.4% S, and 4.4% Cu concentrate at 9% solids at 200xc2x0 C. and 200 atmospheres oxygen. The molybdic acid precipitate that formed during leaching was dissolved using ammonia for recovery of an ammonium molybdate.
2 Sada, Koji, xe2x80x9cExtraction of molybdenum,xe2x80x9d Japanese patent 15.207(""62), assigned to Awamura Mining Co., Ltd. 
In another process disclosure3, alkali hydroxide and alternatively ammonium hydroxide was added continuously to the aqueous solution to neutralize the acid as it formed and maintain the pH at 7-12. Other authors4 postulated the formation of a molybdenum-iron heteropoly complex that decomposes as the acid concentration increases. It also may be a ferrous complex that decomposes as the soluble iron is oxidized to ferric.
3 Hallada, Calvin J., et al., xe2x80x9cConversion of molybdenum disulfide to molybdenum oxide,xe2x80x9d German patent 2,045,308 (1971). 
4 Mel""nikov, B. S. and Shapiro, K. A., xe2x80x9cWater-autoclave decomposition of molybdenite raw material,xe2x80x9d Protsessy Poluch. Rafinirovaniya Tugoplavkikh Met. (1975) 113-120, 253-260. 
Early applications of molybdenum solvent extraction utilized tertiary amine to extract the molybdenum solubilized by sodium hydroxide leaching of roasted molybdenite calcines. Secondary amines and quaternary ammonium compounds extract anionic molybdenum using a similar chemistry. Amines also were used for extraction of molybdenum from molybdenite roaster scrubber solutions.
Molybdenum is an impurity in many uranium ores. When uranium ores are acid leached, some molybdenum reports to the acid leach solution. The tertiary amines readily available during the 1950xe2x80x2s and early 1960xe2x80x2s tended to have an amine-molybdenum complex with poor solubility in aliphatic diluents (kerosene).
Several Russian researchers worked with acid leaching of oxide ores. In many cases the acidity was sufficient for most of the molybdenum to be in a cationic form. Therefore, the cation exchanger (di, 2, ethylhexyl phosphoric acid (DEHPA)) received much study regarding the recovery of molybdenum from complex acid solutions. Karpacheva et al.5 determined that in acid solutions the molybdenum was not present as the simple molybdenyl cation but, the molybdenum was present as polymeric cations. The co-extraction of iron is a major problem when using DEHPA. The authors noted that in a nitric acid system, the acid concentration needs to equal or exceed 3 molar to prevent significant iron extraction. Other authors6 reported on the
5 Karpacheva, S. M. et al., xe2x80x9cExtraction of molybdenum and iron (III) by di-2-ethylhexyl hydrogen phosphate,xe2x80x9d Russian Journal of Inorganic Chemistry, V 12, 7, p 1014-1016 (1967). 
6 Chiola, Vincent, xe2x80x9cSeparation of molybdenum values from tungsten values by solvent extraction,xe2x80x9d U.S. Pat. No. 3,607,008 (1971). benefit of modifiers in reducing the iron coextraction, e.g. tributyl phosphate, dibutyl butyl phosphonate. 
Palant et al.7 made a detailed study of the extraction of molybdenum by DEHPA. The solutions studies were prepared by dissolving MoO3 in sulfuric acid, hydrochloric acid, or nitric acid solutions.
7 Palant, A. A. et al., xe2x80x9cExtraction of molybdenum (VI) with bis(2-ethylhexyl) hydrogen phosphate from an acidic medium,xe2x80x9d Inst. Metall. im. Baikova, Moscow, USSR, Report deposited (1979) pp. 1-19. 
Amine exchange has also received much study during the past 40 to 50 years. The difficulties presented by the poor solubility of the amine-molybdenum complex were addressed by using aromatic diluents. Macinnis et al.8 used tri-n-capryl amine (ALAMINE 336) with the aromatic diluent #289. The authors discuss amine extraction of a complex sulfate-bearing anion. At pH values of 3 and higher, they determined that the ion exchange type mechanism shown in Equation I below predominates.
2MoS2+6H2O+9O2xe2x86x922H2MoO4↓4H2SO4xe2x80x83xe2x80x83(I) 
At pH values below 3, they postulate that the following occurs along with Equation I.
n(R3NH.HSO4)+(MoxOyHz)n.(SO4)mxe2x86x92(R3NH)n.(MoxOyHz)n.(HSO4)n.(SO4)mxe2x80x83xe2x80x83(II) 
8 MacInnis, M. B., Kim, T. K., and Laferty, J. M., xe2x80x9cThe use of solvent extraction for the production and recovery of high-purity ammonium paramolybdate from normal alkali molybdate solution,xe2x80x9d First Intl Conf on Chemistry and Uses for Molybdenum, p. 56-58 (1973). 
9 Aromatic diluent #28 is a solvent from 1960 available from Missouri Solvents and Chemicals. The solvent had a boiling range of 165 to 193xc2x0 C., a flash point of 122xc2x0 F., a Kauri butanol value of 73, and was 74% aromatics. 
Equation II infers that (MoxOyHz)n.(SO4)m is not ionized and MacInnis postulates some cation transfer. This postulation is based on the fact that 35S tagged sulfur was found to transfer both from the organic to the aqueous phase and from the aqueous to the organic phase.
Litz found in 1970, that tris, tridecyl amine could be used successfully for molybdenum solvent extraction with an aliphatic diluent. There still was potential for formation of insoluble molybdenum-amine complexes, but the molybdenum-tris tridecyl amine complex""s solubility in the diluent was much higher than with other tertiary amines. Tris tridecyl amine in an aliphatic diluent was used in a number of pilot circuits for molybdenum solvent extraction from roaster scrubber solutions and to recover byproduct from uranium leach solutions, but it may never have been used in a commercial circuit.
The transfer of sulfate from strongly acid solutions is a problem with using amines. Also, the amines are relatively nonselective and will transfer silicon, phosphorus, and arsenic probably as heteropoly compounds.
The coextraction of silicon and subsequent solids precipitation during stripping has been a major problem during other studies. The silicon problem was addressed by filtration of the first stripping stage mixture prior to advancing to the settler. Sulfate transfer was high because the solvent could not be fully loaded with molybdenum, i.e., to avoid diluent-insoluble molybdenum-amine complexes, the sulfate transfer was large.
Efficient recovery of chemical-grade ammonium dimolybdate (ADM) requires high purity feed solutions containing 200 to 230 g Mo per liter. Impurities in the solution must be removed to avoid inclusion in the ADM. Impurities, that form hydroxides or sulfides, can be removed by additions or pH-control. Other impurities will build up and unless the mother liquor is bled from the crystallization will report to the ADM.
Typical molybdenum solvent extraction systems acidulate the feed solution, if necessary, prior to contact with the extractant in the mixer. Generally this means that the extractant is converted to the bisulfate form by acid in the feed solution and then the desired anion exchanges with the bisulfate. When molybdenum is acidulated there is potential for localized high acid concentrations that can form sulfate-bearing molybdenum species.
It is an object of the present invention to provide an integrated process for producing high purity ammonium dimolybdate or molybdenum oxide through a process that includes the pressure oxidation of low grade molybdenite concentrates or molybdenum intermediates.
It is a further object of the present invention to provide an improved molybdenum pressure oxidation process which produces a high purity product at reduced capital and operating costs.
It is a further object of the present invention to provide an improved solvent extraction method which rejects sulfate and metallic impurities by extracting the molybdenum in an ionic form that contains no sulfate.
The objects set forth above as well as further and other objects and advantages of the present invention are achieved by the present invention now described in summary fashion and with further examples below in preferred embodiments of the practice of the invention.
The present invention provides a process of producing a high purity ammonium dimolybdate or molybdenum oxide through the pressure oxidation of low grade molybdenite concentrates or molybdenum intermediates. The process entails oxidizing the molybdenite concentrates or intermediates in an autoclave operating at greater than 50 p.s.i. oxygen overpressure, preferably between 80-120 p.s.i., at a temperature greater than 200xc2x0 C., preferably between 210-220xc2x0 C. to effect almost complete oxidation of the concentrate while optimizing the process chemistry and autoclave conditions to solubilize as little of the molybdenum values as possible. A method of maximizing the insoluble molybdenum values is disclosed in U.S. patent application entitled xe2x80x9cAutoclave Control Mechanisms for Pressure Oxidation of Molybdenitexe2x80x9d which is incorporated by reference herein (and a copy of which is provided at Appendix A hereto). The resulting autoclave discharge has greater than 99% of the molybdenum concentrates oxidized and greater than 80% of the molybdenum values insoluble.
The autoclave discharge is then subjected to an alkaline leaching of the POX residue using sodium carbonate and sodium hydroxide. More than 99% of the molybdenum dissolves. The molybdenum in this alkaline solution is recovered readily using a secondary amine solvent, ditridecyl amine (DTDA). The molybdenum is loaded into the organic phase at 4.0 to 4.5 pH. The molybdenum-loaded organic is stripped with ammonium hydroxide to produce solutions suitable for recovery of chemical-grade ADM and ultimately chemical-grade molybdenum oxide.
Alternatively, the autoclave discharge may be subjected to an ammoniacal leaching of the POX residue. More than 99% of the molybdenum dissolves. Most of the cosolubilized impurities are precipitated from the ammoniacal leach solution. The solution is evaporated to crystallize chemical grade ADM. An additional route is provided when the leach solution contains more sulfate than is desirable for crystallization of chemical-grade ADM, producing a product suitable for technical grade ADM and ultimately technical grade molybdenum oxide.
Alternatively, the POX residue may be subjected to an alkaline leach with sodium carbonate and sodium hydroxide before a liquid-solid separation step such that all of the molybdenum is soluble and the copper and iron transfer to the solids. More than 98% of the molybdenum is transferred to the filtrate from this neutralization. The molybdenum is readily recoverable using the DTDA solvent extraction process. The ammoniacal strip solutions from the DTDA extraction are suitable for recovery of chemical-grade ADM and ultimately chemical-grade molybdenum oxide.
Cementation by scrap iron can be used to readily recover the copper either from the raffinate produced from molybdenum solvent extraction of the POX leach solution or directly from the POX leach solution. Gold and silver values transfer to the final leached solid residue produced by each embodiment and are recyclable to a copper smelter. Most of the rhenium, arsenic, and phosphorus are dissolved regardless of the leaching conditions.
Other objects, features and advantages of the invention will be apparent from the following description of preferred embodiments thereof, including illustrative non-limiting examples of the practice of the process.